Use configure_lit_site_cfg instead of configure_file.
configure_lit_site_cfg defines some more parameters that are used in
lit.site.cfg.in. configure_file would leave those empty. These additional
definitions seem to be unimportant for regression tests, but unittests do not
work without them.
In case of out-of-tree builds, define the additional parameters with default
values. These may not take all configuration parameters into account, as
configure_lit_site_cfg would.
git-svn-id: https://llvm.org/svn/llvm-project/polly/trunk@279733 91177308-0d34-0410-b5e6-96231b3b80d8